Ken Rice

Signatures of embedded planets

Collaborators - Kenny Wood (St Andrews), Phil Armitage (JILA/University of Colorado),
                           Barbara Whitney (SSI Colorado), Jon Bjorkman (University of Toledo).

A sufficiently massive planet embedded within a circumstellar disc will transfer angular momentum to the disc material producing a gap. If there is enough time then the gap may be cleared all the way to the inner edge of the disc. Such disc gaps may manifest themselves as deficiencies in the spectral energy distributions (SEDs) of protoplanetary accretion discs. Photons from the central star (and in some cases viscous accretion luminosity) are reprocessed as they pass through the cooler accretion disc producing an excess emission at infra-red wavelengths. If material is missing, as in the case of a gap cleared by an embedded planet, the flux at certain wavelengths will be lower than if the material were present.

ZEUS simulation of a 2 Jupiter mass planet clearing a gap in a protoplanetary accretion disc.

The SED of the classical T Tauri star GM Aurigae shows a deficiency in the near infra-red excess suggesting the presence of a gap in the disc extending out to approximately 4 au. We use smoothed particle hydrodynamics (SPH) to simulate an protoplanetary accretion with parameters appropriate for GM Aurigae to see if density profile resulting from the presence of an embedded planet can produce an SED (computed using a Monte Carlo radiation transfer code developed by Kenny Wood) that matches that observed for GM Aurigae. We consider planets of various masses and find that there is the possibility of the SED constraining the planet mass.

GM Aurigae's SED computed using the azimuthally averaged density profile due to a 1.7 Jupiter mass planet orbiting at 2.5 au (solid line). The dotted line is the input stellar spectrum, the open squares are data points, and the dashed line is an SED computed assuming an equivalent disc in the absences of a gap (click figure for a slightly better quality version). GM Aur's SED computed using density profiles due to embedded planets of various masses. Although all the computed SEDs fits most data points well there is a suggestion that the planet mass must be such as to produce a gap that still contains a reasonable amount of mass. (click figure for a slightly better quality version).

Constraints on a planetary origin for the gap in the protoplanetary disc of GM Aurigae, W.K.M. Rice, K. Wood, P.J. Armitage, B.A. Whitney & J.E. Bjorkmann, MNRAS, 342, 79 (2003)

Ongoing accretion

An interesting property of a number of systems showing near-IR deficits (often referred to as transition objects) is that matter is still accreting onto the central star. If the inner disc is largely clear of material, then this isn't possible. This ongoing accretion, together with the deficit of near-IR flux, therefore suggests that somehow the amount of dust in the inner disc has been significantly reduced relative to the amount of gas. One possibility is that a planet in the disc has opened an annular gap, but has been unable to completely clear the inner disc of gas. The pressure gradients at the outer edge of the gap will, however, act to prevent some dust grains from migrating into the inner disc. This could then produce an inner disc with a reasonably large gas density - allowing for ongoing accretion - but with a very low dust density - producing the near-IR deficit. One anomaly is CoKu Tau/4 which has a near-IR deficit, but does not show any accretion. One possibility is that in this system, the companion is quite massive, producing an inner disc with very little gas or dust. It has just been discovered that this is indeed the case as CoKu Tau/4 is actually a binary system.

Dust filtration at gap edges: implications for the spectral energy distributions of discs with embedded planets, W.K.M. Rice, P.J. Armitage, K. Wood & G. Lodato, MNRAS, 373, 1619 (2003)